1. Field of the Invention
The present invention relates to a mass spectrometer and an ion detector used therein.
2. Description of the Related Art
U.S. Pat. No. 6,091,068 discloses an ion detector that includes a Faraday cup and a tube-shaped continuous-dynode electron multiplier. (Details of a tube-shaped continuous-dynode electron multiplier are disclosed in U.S. Pat. No. 5,866,901.) In a Faraday cup mode of operation, the Faraday cup is connected to the input of an electrometer. The incoming ion beam formed from positively charged ions impinges on the collector plate of the Faraday cup. The ions are neutralized upon striking the collector plate, drawing a current as a signal output to the electrometer.
The continuous-dynode electron multiplier in U.S. Pat. No. 6,091,068 includes a conical entrance opening. A grid shield is positioned adjacent to the conical entrance opening. During an electron multiplier mode of the ion detector, a high electrical potential is established at the grid shield so that incoming ions are drawn into the. conical entrance opening. At this time, readings are taken from the output of the continuous-dynode electron multiplier.
Continuous-dynode electron multipliers cannot be used with a heavy current, so have a limited dynamic range of 0.1 FA to 100 nA. As shown in FIG. 1, Faraday cups have a dynamic range of only about 1 mA to 1 xcexcA. Therefore, there is a range Y where the ion detector of U.S. Pat. No. 6,091,068 cannot take accurate readings.
Also, continuous-dynode electron multipliers only have a small secondary electron emissive surface for multiplying electrons. The surface area of the secondary electron emissive surface is limited by the inner surface of the channel running through the tube. The channel is an approximately 1 mm diameter hole, so the electron density per unit surface area is great. Therefore, a large burden is placed on the secondary electron emissive surface in the channel so that the continuous-dynode electron multiplier has a short life.
It is an objective of the present invention to overcome the above-described problems and provide an ion detector with a broad dynamic range and with a long use life.
In order to achieve the above-described objectives, an ion detector according to the present invention includes an ion input face, a Faraday cup, an ion-to-electron converter dynode, two ion deflection electrodes, an electron multiplier portion, and an anode. The ion input face is formed with an ion input opening. The Faraday cup has an ion collection surface that confronts the ion input opening. The ion-to-electron converter dynode is disposed to one side with respect to the Faraday cup and the ion input opening and has a conversion surface that converts impinging ions into electrons. The two ion deflection electrodes generate an electron lens that attracts and focuses ions from the ion input opening toward the conversion surface of the ion-to-electron converter dynode. The electron multiplier portion receives and multiplies the electrons from the ion-to-electron converter dynode, and includes a plurality of dynodes that multiply electrons one after the other. The plurality of dynodes are juxtaposed in an arc-shape around the Faraday cup. The anode receives electrons from the electron multiplier portion and outputs a signal that corresponds to the amount of input ions.
A mass spectrometer according to the present invention includes the above-described ion detector, an ionization portion, and a mass separator. The ionization portion converts molecules of a sample into ions. The mass separator separates desired ions from other ions from the ionization portion. The ion input face confronts the mass separator and the ion collection surface of the Faraday cup confronts the mass separator through the ion input opening.
According to another aspect of the present invention an ion detector includes an ion input face, a Faraday cup, an ion-to-electron converter dynode, an ion deflection electrode, an electron multiplier portion, and an anode. The ion input face is formed with an ion input opening. The Faraday cup has an ion collection surface that confronts the ion input opening. The Faraday cup is connected to ground. The ion-to-electron converter dynode is disposed to one side with respect to the Faraday cup and the ion input opening. The ion-to-electron converter dynode is applied with a high voltage and has a conversion surface that converts impinging ions into electrons. The ion deflection electrode generates, with the Faraday cup and the ion-to-electron converter dynode, an electron lens that attracts and focuses ions from the ion input opening toward the conversion surface of the is ion-to-electron converter dynode. The electron multiplier portion receives and multiplies the electrons from the ion-to-electron converter dynode. The electron multiplier portion includes a plurality of dynodes that multiply electrons one after the other. The plurality of dynodes are juxtaposed in an arc-shape around the Faraday cup. The anode receives electrons from the electron multiplier portion and outputs a signal that corresponds to the amount of input ions.